The Nationals placed Adon on the 15-day injured list Wednesday with a right biceps strain, Andrew Golden of The Washington Post reports.
Adon hasn't pitched in nearly a week, so it's unclear when he might have gotten injured. The right-hander has posted a 6.75 ERA and 6:4 K:BB over 10.2 frames for the Nationals this season. Orlando Ribalta was recalled from Triple-A Rochester to take Adon's spot in the bullpen.